Lab talk

Nov 19, 2009

New routes to adjusting light from artificial atoms

Until recently, the relationship between the intrinsic symmetry and the optical response of silicon quantum dots has been overlooked. Relying on the general concept of symmetry lowering, recent calculations indicate new effective routes to modulate the optical response of these tiny structures without affecting their stability.

Advances in synthetic methods have made possible the efficient production of silicon dots with various sizes, shapes and core structures. The exhibited photoluminescence shows a tremendous potential for applications in the area of energy. However, an important challenge is adjusting the optical response by manipulating the electronic states around the last occupied, first empty levels. In crystals this is usually achieved by doping but this route proves to be difficult at the nanoscale.

In a recent study published in the journal Nanotechnology, researchers from the University of Minnesota have focused on highly symmetrical dots, sometimes called "artificial atoms". As in atoms, high symmetry in silicon dots brings electronic degeneracies and large level spacings, and enforces strict selection rules for the optical transitions between levels. Many transitions are forbidden. For instance, the energy spacing between last occupied, first empty levels is generally different from the first possible excitation. Of course no symmetry implies no degeneracy and all transitions would be allowed.

The question addressed by the team was whether it is possible to alter the atom-like electronic levels of such dots without considering the unlikely endohedral doping. In atoms, splitting the degenerate energy levels is usually accomplished by breaking the symmetry with the help of an external magnetic field. In silicon dots, the researchers demonstrated via density-functional theory calculations that symmetry lowering and level splitting could be readily accomplished in new ways. For example, introducing a slight structural imperfection vis-à-vis the spherical shape (see image) by applying mechanical squeezing and contaminating the surface with sodium atoms.

The continual development of nanotechnology will provide a greater range of highly symmetrical silicon quantum dots. The uncovered connection between symmetry and electronic states makes these structures very exciting for both fundamental and applied research. Higher-level calculations are under way to quantify the energy of the emitted light more precisely. In optoelectronics, symmetry lowering could become a useful strategy for manipulating the optical response.

About the author

The work was performed at the University of Minnesota and supported primarily by the local MRSEC Program of the National Science Foundation under the IRG 4: Nanoparticle-Based Materials. Additional support from the National Science Foundation CAREER and NIRT Programs and from the American Chemical Society Petroleum Research Fund is greatly acknowledged. Calculations were performed at the Minnesota Supercomputing Institute. Dong-Bo Zhang is a PhD student studying Materials Science at the University of Minnesota. Prof. Traian Dumitrica is the head of the Computational Nanomechanics Group in the Department of Mechanical Engineering at the University of Minnesota.